Mass spectrometry



Patented Mar. 17, 1953 MASS SPECTROMETRY Harold W. Washburn, Pasadena,and Clifford E.

Berry, Altadena, Calif., assignors to Consolidated EngineeringCorporation,

Pasadena,

Calif., a. corporation of California Application August 3, 1950, SerialNo. 177,502

Claims. (Cl. 250-4L9) This invention relates to mass spectrometry andparticularly to mass spectrometry wherein charged particles of difieringmass-to-charge ratio are segregated according to their periodicity ofmotion in a magnetic field.

Mass spectrometry in general involves ionization of a sample to beanalyzed, as by bombardment with an electron beam, and segregation ofthe resultant ions in accordance with their massto-charge ratio. Themeasured magnitude of the current developed by discharge of the ions ofthe given mass-to-charge ratio provides a basis for calculating thepartial pressure of those molecules in the sample from which theseparticular ions were derived. Where a sample is to be analyzed for morethan one component, the practice is to scan the mass spectrum bysuccessively discharging and measuring ions of different massto-chargeratio. The spectrum is scanned by varying one or more of the parameterswhich determine the paths of travel of the ions.

One means of segregating or sorting the ions comprises introducing theminto or forming them in a space traversed by a magnetic field and a highfrequency alternating field normal to the magnetic field. It has beenfound that ions will assume characteristic paths of motion in the spacetraversed by these fields, the orbits of which depend primarily uponwhether or not the ions are in resonance with the alternating field. Atany given magnetic field strength and at any given frequency of thealternating field, only ions of a given mass-to-charge ratio will be inresonance with the alternating field.

Under these circumstances, the so-called resonant ions will pursue apath from their origin in the form of an ever expanding spiral ofuniformly increasing radius, the radius being normal to the magneticfield. The non-resonant ions, on the other hand, will be driven in anexpanding and contracting spiral, i. e. the radius of travel willincrease in decreasing increments to a maximum and will thereuponcommence to decrease in increasing increments so that the ion pathcollapses back to the origin. The origin may be considered as the axisof ion flow into the space, or as the axis of an ionizing electron beamprojected through the space parallel to the magnetic field.

As might be expected, the non-resonant ions of mass most closelyadjacent the mass of the resonant ions will have the largest orbitalradii, and for this reason the degree of resolution depends upon theselectivity with which the resonant ions are measured to the exclusionof the non-resonant ions of closely adjacent mass-tocharge ratio. Toscan the mass spectrum, it is only necessary to vary the frequency ofthe alternating field so that ions of a different mass-tocharge ratiowill become in resonance therewith.

It can be shown that all ions of resonant mass starting at arbitrarytimes at the origin and with no initial energy, lie in a roughly radialband.

In other words, the resonant ions orient themselves into a radial spokewith the axis of rotation coinciding with the axis of origin. Phasedifferences with respect to the time of formation are substantiallyeliminated within the first few revolutions of the spiral path. Inpractice the origin may be anywhere along the ionizing electron beam. Asa result of initial energies and stray fields ions may move parallel tothe beam, a small retaining D. C. field generally being applied to limitthis movement within the confines of the crossed fields. Thelongitudinal movement does not afiect the characteristic spiral paths ofthe ions, and at any given instant all of the ions of resonant mass willlie in a radial plane intersecting the beam, the beam being the axis oforigin as well as the axis of rotation.

Non-resonant ions, that is, ions whose massto-charge ratio does not fitthe condition 1/ B (l) where:

q=the charge of the ion;

m=the mass of the ion;

w=the angular frequency of the alternating field; and

B=the transverse magnetic field;

will also move in bands. However, the non-resonant bands are bent in acurvilinear configuration and do not lie on a radius of the axis oforigin as does the band of resonant ions. Since the effect of initialenergy in either the ions of resonant mass or non-resonant mass is onlyto obscure or spread the boundaries of the respective bands leavingtheir angular orientation unaffected, these do not require specialconsideration in this instance.

As mentioned above, the resolution depends upon the selectivity withwhich the resonant ions are discharged with respect to the non-resonantions of closely adjacent mass. From the foregoing considerations, it isseen that resonant ions do not continuously strike a collector electrodewhich may be located in the field and spaced from the axis of origin forthe reason that the resonant ions are disposed in a radial spoke andwill hence strike the collector electrode only once for each revolutionof the spoke. Non-resonant ions of adjacent mass will interfere with theresolution of the instrument by discharging at the electrode in theperiods intervening between successive impingement of the spoke ofresonant ions on the electrode. Theoretically, by placing the collectorelectrode far enough from the axis of origin, nonresonant ions will notachieve a sufficient radius of trave1 to attain the collector electrode.How ever, in practice, the ideal situation is not achieved and somenon-resonant ions will discharge at the collector interfering with theresolving power of the instrument.

We have now discovered a method for increasing the resolving power of amass spectrometer of this type by an appreciable factor, which method isbased upon the discovery that the resonant ions will travel in a radialspoke as described.

Thus, the invention contemplates in mass spectrometry involving theformation of ions and the separation thereof in an analyzer region bycausing them to travel in different paths under the influence of amagnetic field and a transverse alternating electrical field, andmeasuring ions by means of a collector electrode, the improvement whichcomprises maintaining the collector electrode and the alternatingelectrical field in matched relationship whereby only ions of a givenmass-to-charge ratio will be measured at the collector electrode.

To accomplish this method, the collector electrode and associatedamplifier and recorder are electrically gated so that the electrode isresponsive to ion discharge only during a small fraction of the periodof operation, the sensitive period being synchronized with thealternating electrical field so as to coincide with the instant at whichions of resonant mass impinge on the collector electrode. By sooperating the collector electrode, all ions which impinge thereon in thecomparatively longer period during which the ions of resonant mass arerevolving through an angle of approximately 360 will not be sensed. Inthis way, non-resonant ions of closely adjacent mass can be excludedfrom the collection and amplification system and the resolution of theinstrument may be increased by a comparatively large factor.

Our invention contemplates improvements in instrumentation whichfacilitate the practice of the method of the invention involving in amass spectrometer the combination which comprises an analyzer chamber,means for ionizing a gas sample introduced into the chamber, collectormeans disposed in the chamber for discharging ions impinging thereon,amplification means connected to the collector, means for amplifyingdischarge current developed at the collector means. electrodes disposedat opposite ends of the chamber, a high frequency oscillator connectedto the electrodes to establish a high frequency alternating field acrossthe chamber, means for establishing a magnetic field across the chambernormal to the alternating electrical field and means in association withthe collector means for rendering the same insensitive to ion dischargeduring the period in which ions of a given mass-tocharge ratio do notstrike the collector means.

Many means may be employed for controlling the sensitivity of thecollector electrode and amplification system. One convenient meanscomprises gating the collector in synchronism with the high frequencyalternating field responsive to an impulse delivered to the collectorand amplification system from the high frequency oscillator.

The invention will be more clearly understood with reference to thefollowing detailed description taken in conjunction with theaccompanying drawing wherein:

Fig. 1 is a schematic diagram of one form of instrument in accordancewith the invention; and

Fig. 2 is a circuit diagram showing in greater detail one means ofgating the collector and amplification system of the apparatus of Fig. lin synchronism with the high frequency electrical field.

The apparatus of Fig. 1 includes an envelope H) having an exhaust line Hfor connecting the envelope to an evacuating system (not shown) and asample inlet line I2 for introduction of a gas sample to be analyzed. Anopen ended conductive box 14 is disposed within the envelope andelectrodes l6, I! are mounted at opposite ends of the box. Theelectrodes l6, I! are insulated from the box, as by the illustrated gap,and substantially enclose the open ends of the box. The sample inletline 12 connects to an inlet chamber 18 formed on an outside wall of thebox l4 midway between the electrodes 16 and I1. An opening IS in thewall of the box gives access from the inlet chamber [8 to the interiorof the box, and a similar opening 20 in the opposite wall of the boxpermits an electron beam 23 to be directed across the box from a gun 22and into the inlet chamber 18. An electrode 24 propels electrons fromthe gun 22 through the box and into the inlet chamber 18 where theyimpinge and are discharged at a target electrode 26. The same propellingpotential may be developed between the gun and the box itself so as toeliminate electrode 24 if desired.

A collector electrode 30 is mounted through electrode i6 and isinsulated therefrom. The collector electrode is grounded through aresistor 32 and is connected to an amplifying and recording network 34.Operation of the collector electrode at ground potential is notnecessary to the practice of the invention.

A high frequency oscillator 36 is connected across electrodes I6 and Hto develop in the analyzer region defined by the box l4, a highfrequency alternating electrical field transverse to the electron beam.Magnetic pole pieces 38, 38 are mounted outside the envelope [4 anddevelop a magnetic field across the analyzer region parallel to theelectron beam. A power supply network 42 is connected to the variouspropelling electrodes, electron gun and electron target to supply thenecessary potentials thereto.

The oscillator 36 is connected through a trigger circuit 44 and a gatecircuit 46 to the amplification and recording network 34 so as to gatethe collector electrode 30 in the manner and for the reasons abovedescribed.

One circuit by means of which the amplifier and recorder may be gated insynchronism with the alternating electrical field is shown in greaterdetail in the diagram of Fig. 2. Referring to this figure, collectorelectrode 30 is connected to the amplifier 34 and is grounded throughthe resistor 32. A diode 50 is connected to the circuit between thecollector and the amplifier, cathode 50A of the diode being groundedthrough a resistor 52. The collector is hence normally grounded throughthe diode which bleeds any signal from the collector electrode before itis applied to the amplifier 34. Oscillator 36 develops an A. C. signalshown by the trace 36A, which is applied through a step up transformer53 to a phase shift network 54 which shifts the phase of the signal 36Aas shown in MA, this signal being applied to a clipper 56. The phaseshift network is preferably variable as shown so that the relationshipof collector sensitivity to the alternating electrical field may beadjusted at will. The clipper comprises rectifier elements 51, 58 and abias battery 59 biasing the rectifier element 58. The output signal fromclipper 56 as represented by the trace 56A is difierentiated in adifferentiating network 62 to convert it into a series of pips asillus'- trated at 62A. The signal 62A is in turn rectified in arectifier element 64 erasing the negative pips to give a signalillustrated by the trace 64A. The output of the rectifier is applied tothe cathode of diode 56 so that during the interval of each of thesepips, voltage developed across resistor 32 responsive to dischargecurrent at the collector electrode will not be shorted through the diodeand will be amplified in the amplifier 34.

The signal applied to the gate tube 50 is so synchronized with thealternating voltage delivered by the oscillator 36 that the collectorand amplification system is sensitive only when the resonant ions are inthe vicinity of the collector electrode and not otherwise. If thecollector electrode is differently oriented in the analyzer region, forexample if it projected into the region at a point 90 from the positionshown, the same gate circuit could be applied with suitable adjustmentof the phase shift to vary the periods at which the collector isenergized to again correspond to the time intervals in which resonantions are in the vicinity of the collector.

The operation of the illustrated apparatus is as follows. A gas sampleintroduced through inlet line it diffuse-s from the inlet chamber intothe analyzer region defined by the box M in substantially an un-ionizedstate. In the analyzer region any gas molecules intersecting theelectron beam 23 are ionized and under the influence of the transversemagnetic and alternating fields are set in motion which is characterizedby rotation around the electron beam. Ions of resonant mass will travelin a spiral path of ever increasing radius and will strike the collectorelectrode so. The conductive walls of the box are conveniently providedwith a slight positiv bias so as to prevent ion discharge at the walls.The efiect of this bias is to cause ions formed along the electron beamto oscillate between the opposite walls of the box. This oscillation,however, does not affect the spiral paths of the ions. During theintervals in which no resonant ions are discharging at the collectorelectrode, the electrode and amplification network are renderedinsensitive by the gate tube 50 so that any nonresonant ions which maystrike the electrode in this interval are not measured.

The effect of the invention is to bring about a time separation as Wellas a space separation between resonant and non-resonant ions, it havingbeen found that the space separation alone is not entirely satisfactorybecause of the closely approximating radii of non-resonant ions of massapproaching the mass of the resonant ions. Particularly is this truewhen the spectrum is scanned by varying the frequency of the alternatingfield. By gating the collector advantage is taken of the phasedifferences between the resonant and nonresonant ions. Moreover, inpassing from one mass peak to an adjacent mass peak as the frequency ofthe alternating field is varied, the bases of the adjacent peaks arenarrowed as a result of the gating and undesirable overlap of theadjacent peaks is minimized.

Another factor which ordinarily has a bearing on the resolution achievedis the existence of ions having small initial energies. These ions tendto spread the ion bands so that ions having an initial energy and ofmass adjacent the resonant mass may strike the collector electrode.However, such initial kinetic energy does not alter the phaserelationship between the resonant and nonresonant masses and by gatingthe collector as described some of the non-resonant ions having initialkinetic energy may be excluded from the measurement.

We claim:

1. In a mass spectrometer the combination which comprises an analyzerchamber, a collecting system including a collector electrode disposed inthe chamber for discharging ions impinging thereon a sensing circuitconnected to the collector, means including a high frequency oscillatorfor establishing an alternating electrical field across said chamber,means for establishing a. magnetic field across said chamber transverseto the alternating electrical field, and means in association with saidcollecting and sensing systems to render the sensin circuit insensitiveto ion discharge at the collector electrode for a part of each cycle ofthe alternating field.

2. In a mass spectrometer the combination which comprises an analyzerchamber, a collecting system including a collector electrode disposed inthe chamber for discharging ions impinging thereon a sensing circuitconnected to the collector, means including a high frequency oscillatorfor establishing an alternating electrical field across said chamber,means for establishing a magnetic field across said chamber transverseto the alternating electrical field, and means for matching the sensingcircuit with the oscillator for rendering the sensing circuitinsensitive to ion discharge at the collector electrode for a part ofeach cycle of the alternating field.

3. In a mass spectrometer the combination which comprises an analyzerchamber, a collecting system including a collector electrode disposed inthe chamber for discharging ions impinging thereon a sensing circuitconnected to the collector, means including a high frequency oscillatorfor establishing an alternating electrical field across said chamber,means for establishing a magnetic field across said chamber transverseto the alternating electrical field, and means for matching the sensingcircuit with the oscillator for rendering the sensing circuitinsensitive to ion discharge at the collector electrode for a major partof each cycle of the alternating field.

4. In a mass spectrometer the combination which comprises an analyzerchamber, a collecting system including a collector electrode disposed inthe chamber for discharging ions impinging thereon a sensing circuitconnected to the collector, means including a high frequency oscillatorfor establishing an alternating electrical field across said chamber,means for establishing a magnetic field across said chamber transverseto the alternating electrical field, and a gate circuit connectedbetween the oscillator and the sensing circuit for rendering the sensingcircuit insensitive to ion discharge at the collector electrode for allbut a fraction of each cycle of the alternating field.

5. In a. mass spectrometer including an analyzer region, means forestablishing across the analyzer region an alternating electrical fieldand a transverse magnetic field, the combination comprising a collectorelectrode disposed in the region for discharging ions thereon, a sensingcircuit connected to receive discharge signals from the collectorelectrode, and means rendering the sensing circuit insensitive todischarge signals from the collector electrode during a part of eachcycle of the alternating electrical field.

HAROLD W. WASHBURN. CLIFFORD E. BERRY.

No references cited.

